1. Field of the Invention
The present invention relates to an optical storage disk which can store information at an extremely high storage density and a recording and reproducing system using such optical storage disks.
2. Prior Art
An information reproducing system has been devised and demonstrated in which the information prerecorded on an optical storage disk is reproduced by focusing a coherent light beam such as a laser beam into an extremely fine spot on the disk and scanning or tracking the recording tracks, thereby reproducing the prerecorded information. Such prerecorded optical storage disks have been available on the market as "VIDEO DISK". In order to store or record information or data, holes are formed in a disk so that the prerecorded information can be detected in terms of the difference in phase between the light reflected from such formed hole and the light reflected from the disk in places where a hole has not been formed.
There have been also devised and demonstrated various types of optical storage disks on which the user can record any desired information that he or she wants. The surface of such disk is deposited with a thin film of a photosensitive recording medium whose optical properties are changed when exposed to the spot of a light beam. In the near future such optical storage disks will be widely used as data files and will be developed into recordable optical video disk system in which information can be recorded. At present, thin recording films comprise metals such as Te, tellurium suboxides TeOx (0&lt;x&lt;2) or MnBi which exhibits opto-magnetic properties. In the case of the thin metal film, holes are formed by the laser beam; in the case of the thin TeOx film, the optical properties of a spot exposed to the laser beam are changed; and in the case of the thin film of an opto-magnetic medium, the rotation of polarizing angle is used for the recording and readout.
One of the typical commercially available, replay-only optical video disks is 30 cm in diameter and has recording tracks spaced apart from each other by 1.67 .mu.m. If the NTSC system is used for prerecording and reproducing the video signal on and from such disk, a program of motion picture can be displayed for 30 minutes by spinning the disk at 1800 rpm and scanning the recording track with the spot of a HeNe laser which is coherent. However, in practice, the display time of only 30 minutes is too short, so that there has been strong demands for optical video disks capable of displaying a longer time. There has been already proposed an improved optical video disk which has a display or reproduction time twice as long as that of the conventional disk. In this disk, other recording tracks are interposed between the recording tracks so that the crosstalk problem can be reduced to a minimum level.
One method for minimizing the crosstalk problem is to vary the depths of hole pits between the adjacent recording tracks. For instance, according to the disclosure of Japanese Laid Open Patent Application No. 12805/1979, first tracks in which pits have a depth of 1.8 .mu.m and second tracks in which pits have a depth of 2.0 .mu.m are alternately arrayed or interlaced and a first laser beam of a wavelength of 800 nm is used to scan the first tracks while a second laser beam 900 nm in wavelength is used to scan the second tracks. With the laser beam 800 nm in wavelength, the difference in phase between the ray of light reflected from the pit 2.0 .mu.m in depth and the ray of light reflected from the undisturbed place becomes an integer multiple of the wavelength. Therefore, the pits of 2.0 .mu.m in depth become an optical flat surface to the laser beam 800 nm in wavelength. Therefore, the recorded data can be read out in terms of the difference in phase between the ray of light reflected back from the pit of 1.8 .mu.m in depth and the ray of light reflected back from the undisturbed place. The readout with the laser beam 900 nm in wavelength is substantially similar to that described above. However, this system has a defect that stable control for focusing the spots of two laser beams cannot be ensured because the pits are too deep. In order to vary the depth of pits as described above, photoresist films are used and the intensity of a laser beam is varied depending upon the desired depth. As a result, it is extremely difficult to control the depth of each pit with the desired degree of accuracy.
Japanese Laid Open Patent Application No. 136303/1979 teaches to use a laser beam of a wavelength of .lambda. to scan or track first and second recording tracks which are alternately interlaced, the first tracks having pits of .lambda./4 in depth while the second track, pits of .lambda./8 in depth. First and second photodetectors are used and spaced apart from each other by a predetermined distance in the direction in parallel with the recording tracks in order to intercept the rays of light reflected back from the disk. The signal recorded along the second tracks with the pits of .lambda./8 in depth is detected in terms of the difference between the outputs from the first and second photodetectors while the signal recorded along the first tracks with the pits of .lambda./4 in depth, in terms of the sum of the outputs from the first and second photodetectors. However, it is still extremely difficult to form such pits with a desired degree of dimensional accuracy and consequently the above-described system is unsatisfactory in practice.
In the case of the real-time recordable optical storage disk, it is almost impossible to change the recording or reproduction method between the adjacent recording tracks. At present, a maximum motion-picture display or reproduction time of the optical video disk of a diameter of 30 cm is only 30 minutes.
In the recording-and-reproducing system with the optical storage disk, semiconductor lasers have become widely used because they are compact in size and the direct power modulation is possible. For the recording of data, a high-output laser beam must be used which has the wavelength in the near-infrared radiation range. With such a laser beam, it is impossible to focus a spot whose diameter is as small as that of the spot of a HeNe laser beam (0.633 .mu.m in wavelength). It is possible if a lens with a higher numerical aperture NA is used, the laser beam can be focused into a very small spot, but the increase in numerical aperture NA is limited because of the aberration effects which are caused due to the variations in thickness of the optical storage disk. Therefore, it has been impossible to reduce the track pitch or spacing below 1 .mu.m. Consequently, research and development for an increase in storage density on the optical storage disk must be started from a novel underlying principle which is completely different from those of the conventional recording and reproducing systems.